1. Field of the Invention
The present invention is directed to a process for the preparation of optionally cellular polyurea elastomer moldings which are not brittle after demold and cooling by using polyisocyanate semi-prepolymers based on hydroxy polyesters.
2. Description of the Prior Art
U.S. Pat. Nos. 4,433,067; 4,444,910; 4,396,729; 4,530,941; and 4,774,263 are all directed to the production of polyurea elastomers by the RIM process. One of the difficulties of these prior art processes is that the molded articles are brittle after demold and cooling, especially molded articles containing reinforcing agents and/or having a high flex modulus. While it is known that the brittleness of the molded article can be reduced by post curing at elevated temperature, this does not reduce the difficulty of getting the brittle part out of the mold without the part breaking or shattering.
U.S. Pat. No. 4,546,114 proposes one solution to brittle parts by heating the mold to temperatures in excess of 110.degree. C. or maintaining the part in the mold for substantially longer periods than normal to reduce brittleness. However, this solution is impractical on a commercial scale since most existing molds do not have the capability for establishing and maintaining the elevated temperatures necessary and, in addition, the extended period of time which the part must remain in the mold reduces the overall production efficiency.
It is also proposed in the prior art in U.S. Pat. No. 4,442,235 that part brittleness of primarily polyurethane elastomers may be reduced by using 1,3,5-triethyl-2,6-diaminobenzene as the chain extender instead of a mixture of 1-methyl-3,5-diethyl-2,4- and/or -2,6-diaminobenzene.
In order to conduct a reaction injection molding (RIM) process, it is necessary to have liquid reactants and one of the most common methods of liquefying diphenyl diisocyanate (MDI) is the incorporation of ether groups by reaction with either low or high molecular weight polyether polyols. In accordance with the present invention it has been discovered that if these ether groups are replaced by ester groups, that the problem with brittleness can be eliminated, without the need for using excessively high mold temperatures.
It is an object of the present invention to overcome the disadvantages of the prior art with regard to part brittleness at demold and to provide high flexural polyurea elastomer moldings which can be processed using existing equipment, which can be removed from the mold without part breakage and which retain all of the advantageous properties of prior art elastomers. These objects may be achieved in accordance with the present invention as set forth hereinafter.
U.S. Pat. No. 4,374,210 is directed to the use of isocyanate prepolymers or semi-prepolymers as the polyisocyanate component for the production of polyurethane elastomers. This patent discloses and exemplifies (note prepolymer I at column 11/Table 1) the use of prepolymers prepared from hydroxy polyesters for the production of polyurethane elastomers, i.e., elastomers prepared from high molecular weight polyols as opposed to the polyurea elastomers of the present invention which are prepared from high molecular weight amine-terminated polyethers. There is no suggestion in this reference that the problem with part brittleness could be overcome by using polyisocyanate prepolymers or semi-prepolymers prepared from polyester polyols.
U.S. Pat. Nos. 4,201,847 and 4,254,228 both describe an internal mold release agent based on the reaction product of an organic polyisocyanate and a fatty acid ester for use primarily in the production of polyurethane elastomers. These references require the acid component of the polyester to contain a fatty acid; whereas, the acid component of the polyester used in accordance with the present invention is based solely on non-fatty acids.